1. Field of the Invention
The present invention relates to spherical joints and, more particularly, to a spherical joint for coupling three or more links together at one point while allowing the links to be freely movable relative to each other within a predetermined range of motion without allowing any interference between them.
2. Description of the Prior Art
In accordance with the recent trend of increased demand and increased importance of production and machining of high precision parts, production of semiconductors, microsurgery, gene manipulation and cell conformity in a variety of industrial fields, such as a high precision engineering field, a semiconductor manufacturing field, a medical field and a genetic engineering field, the study and development of robots or manipulators for micro-positioning work has been actively carried out.
In the prior art, a variety of serial robots with open links have been used as such manipulators for micro-positioning work in a variety of industrial fields. Due to their open links, such serial robots are somewhat advantageous in that they preferably provide a large workspace, and preferably accomplish improved manipulability. However, these serial robots are problematic in that they inevitably create accumulated errors at their end effectors since they have serial actuators. The serial robots are thus undesirably deteriorated in their operational accuracies. Another problem experienced in the conventional serial robots resides in that their operational performance is undesirably reduced, particularly when they are used in high-speed work or other work with excessively variable weight of dynamic load.
In an effort to overcome such problems experienced in the conventional serial robots, a variety of parallel mechanisms have been actively studied since the 1980s. Such parallel mechanisms have a closed chain structure, and so they are free from actuator-caused errors accumulated at their end effectors in addition to preferably having a high structural strength different from the conventional serial robots, even though the workspace provided by the parallel mechanisms is regrettably smaller than that of the serial robots. The parallel mechanisms thus accomplish a desirably high operational performance when they are used in high-speed work or other work with excessively variable weight of dynamic load. Therefore, it is more preferable to use such parallel mechanisms in place of the conventional serial mechanisms for micro-positioning work.
Such parallel mechanisms are structurally advantageous in a variety of items as described above, and so the parallel mechanisms, particularly, six-degrees-of-freedom parallel mechanisms (for ease of description, referred to simply as xe2x80x9c6dof parallel mechanismsxe2x80x9d herein below) have been preferably and widely used in micro-positioning systems, simulation game systems and a variety of simulators of airplanes and automobiles.
Such a 6dof parallel mechanism is operated by six linearly actuated links, which couple a moving platform to a base platform of the mechanism. In such a 6dof parallel mechanism, the linearly actuated links are coupled to the moving and base platforms using universal joints or ball and socket joints. In order to maximize the potential fields of application and operational performances of such 6dof parallel mechanisms, two or more jointing points are required to be provided at each joints of a moving platform. In addition, it is necessary to design the structure of such joints individually having two or more jointing points such that the joints are not likely to cause interference or limitations to the motion of the linearly actuated links of the parallel mechanisms.
However, a variety of conventionally proposed and used parallel mechanisms having multiaxial joints are problematic in that the design focus of the mechanisms is undesirably limited to the kinetic analysis and the kinetic design of the link mechanisms while disregarding the multiaxial joints, since it is very difficult to optimally design such joints.
As described above, most conventional parallel mechanisms have two or more jointing points at each joint of a moving platform. However, the conventional ball and socket joints or universal joints, typically used as the multiaxial joints of such parallel mechanisms, are designed to couple only two links together, and so the conventional ball and socket joints or universal joints cannot be used for coupling three or more links together at one point. In a multiaxial joint used for coupling two or more links together, the operational performance of the joint directly determines both the size of jointed parts of links and the desired smoothness of motion of the links.
FIG. 1 is a perspective view of a parallel mechanism, with a conventional multiaxial joint having a plurality of stacked pin joints used for coupling six links together. FIGS. 2a and 2b are a perspective view and a partially enlarged view, showing a tetrahedral truss structure with four conventional multiaxial joints coupling three links together at each corner of the tetrahedral truss structure.
As shown in FIG. 1. Kourosh E. Zanganeh, Jorge Angeles, in 1994, proposed a parallel mechanism, of which the moving and base platforms are coupled together by nine links, with six of the nine links being coupled together by one multiaxial joint having a plurality of stacked pin joints (Kourosh E. Zanganeh, Jorge Angeles, xe2x80x9cMobility and Position Analysis of a Novel Redundant Parallel Manipulatorxe2x80x9d, IEEE, IROS, pp. 3043xcx9c3084, 1994).
However, the above-mentioned multiaxial joint with the stacked pin joints has a stacked structure including six pin joints and two ball and socket joints, and so the multiaxial joint undesirably has a large size and a complex construction such that it is very difficult to fabricate the desired joint. In addition, this multiaxial joint is ill-affected by backlash caused by the excessive number of parts. Therefore, the multiaxial joint with such stacked pin joints inevitably causes interference between the links and limitations in the motion of the links due to its large structure.
As shown in FIGS. 2a and 2b, Gregory J. Hamlin and A. C. Sanderson, in 1994, proposed a concentric multilink spherical joint used for coupling a plurality of links together to fabricate a parallel mechanism having a tetrahedral truss structure (Gregory J. Hamlin, A. C. Sanderson, xe2x80x9cA Novel Concentric Multilink Spherical Joint with Parallel Robotics Applicationsxe2x80x9d, IEEE, IROS, pp. 1267xcx9c1272, 1994). As shown in the drawings, this concentric multilink spherical joint has a tri-axial joint structure fabricated with quadric crank links assembled into a plate hinge-type linkage, and is used for coupling three links together at each corner of a desired tetrahedral truss structure.
However, the concentric multilink spherical joint necessarily has six quadric crank links in addition to a plurality of hinge parts for fabricating the six quadric crank links into a desired tri-axial joint structure, and so it is very difficult to produce such a concentric multilink spherical joint. Another problem experienced in the concentric multilink spherical joint resides in that it has a large structure due to the excessive number of quadric crank links and hinge parts, and so the practical fabrication of such a joint is always made difficult.
In order to produce a multiaxial joint having a desired jointing function, it is necessary to design the joint to allow the axes of jointed links to precisely converge at one point, thus accomplishing a precise point contact of the links at the joint. However, the conventional multiaxial joints fail to perform such a desired jointing function, and do not have desired practical utility due to their large and complex structure, in addition to difficulty in fabrication of them.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a spherical joint, which is designed to precisely couple three or more links together at one point while allowing the links to be freely movable relative to each other within a predetermined range of motion without allowing any interference between them.
In order to accomplish the above object, the present invention provides a spherical joint for coupling three or more links together at one point, comprising: a central ball; a hollow spherical body movably covering the central ball without allowing a removal of the ball from the spherical body, with a plurality of upper and lower link insertion openings formed at the upper and lower portions of the spherical body; an upper link inserted into the hollow spherical body through the upper link insertion opening and coupled to the top surface of the central ball; a plurality of link support discs set within the gap between the central ball and the hollow spherical body such that the link support discs are not removable from the spherical body through the lower link insertion openings; and a plurality of lower links inserted into the hollow spherical body through the lower link insertion openings and mounted to the link support discs.
In the spherical joint of this invention, the lower link insertion openings are formed on the hollow spherical body at three positions regularly spaced at angular intervals of 120xc2x0, thus allowing the axes of the lower links mounted to the link support discs to precisely converge at the center of the central ball.
In addition, the hollow spherical body comprises two hollow hemispherical bodies integrated into a desired single spherical body by a locking means, such as locking bolts.
In a brief description, the present invention provides a multiaxial spherical joint, which is used for coupling three or more links together at one point, and comprises a central ball and a hollow spherical body covering a central ball, with one upper link coupled to the central ball and three or more lower links mounted to the link support discs set within the gap between the central ball and the hollow spherical body.